The invention relates to a transdermal therapeutic system containing the active substance scopolamine base and comprising a flexible, active substance-impermeable backing layer, an active substance-containing reservoir layer, a control membrane, a pressure-sensitive adhesive layer for attaching the system onto the skin, as well as a protective film or sheet which is likewise active substance-impermeable and is to be removed prior to application.
Scopolamine is a known substance which is suitable for transdermal application with systemic action, with the aid of a patch. Scopolamine is a so-called antiemetic which is preferably used to avoid nausea and vomitting as, for example, arising from repeated passive changes in balance occurring while travelling.
The therapeutic advantage of transdermal administration consists in that the active substance supply is effected slowly, continuously and in a controlled manner through the transdermal system. Thereby, it is possible to hit the relatively narrow therapeutic window for scopolamine reliably and, on the one hand, to thereby adjust therapeutically effective plasma levels without, on the other hand, having to fear the side effects caused by overdosage, such as, for example, dryness of the mouth, nausea and sensitivity to glare.
U.S. Pat. No. 3,797,494 describes a known transdermal therapeutic system used for administering scopolamine with systemic action. It substantially consists of a backing layer, an active substance reservoir, a microporous membrane, a likewise active substance-containing skin-adhesive layer, and a protective film to be removed prior to use. The reservoir and the skin adhesive layer contain a mixture of polyisobutylenes having various molecular weights and mineral oil. The active substance is dispersed in said mixture as a viscous liquid.
A transdermal system whose active substance-containing components are built up on this basis has, however, considerable disadvantages. Under certain conditions, spontaneous crystallization of the active agent may occur, which has a negative influence on the bioavailability of the active substance in the patch, possibly eliminating it altogether. U.S. Pat. No. 4,832,953 describes in great detail the causes and consequences of such instability. It describes a method making it possible to prevent crystallization by way of subsequent heat treatment of the already packaged patch. The result of this method, however, remains uncertain and uncontrollable. According to the indications given in this document, it is above all scopolamine hydrate which is involved in the unwanted crystallization.
It constitutes a drawback of utmost significance if an active substance contained in a transdermal administration form changes its state of aggregation, under conditions which cannot be defined, at a non-predictable point in time after manufacture, with simultaneous negative influence on bioavailability.
DE-OS 44 38 989 describes a system on the basis of polyacrylate adhesives comprising scopolamine base. In said systems, recrystallization of the active agent is prevented due to the fact that the active substance is present completely dissolved, at a concentration below the saturation limit. The disadvantage here is that the active substance is delivered in vivo with kinetics of the first order and that consequently, in the case of an active substance load of between 1.5 to 2 mg, the permeation rate decreases over the application period of 3 days, and, as a result, is not constant. Only in the case of a higher active substance load would the active substance release be sufficiently constant over the application period, but it still does not take place according to kinetics of zero order. Neither can the problem be solved by the use of membranes as this influences the release rate but not the kind of kinetics.